DETROIT — When Eva Nelson-McClendon first moved to Detroit’s Birwood Street in 1959, she didn’t know much about the wall across the street. At 6 feet tall and a foot thick, it wasn’t so imposing, running as it did between houses on her street and one over. Then she started to hear the talk.Neighbors told her the wall was built two decades earlier with a simple aim: to separate homes planned for middle-class whites from blacks who had already built small houses or owned land with plans to build.“That was the division line,” Nelson-McClendon, now, 79, says from the kitchen of her tidy, one-story home on the city’s northwest side. “Blacks lived on this side, whites was living on the other side. ... That was the way it was.”That’s not the way it is anymore. But the wall remains, a physical embodiment of racial attitudes that the country long ago started trying to move beyond....